NEW YORK >> Many travelers heading to and from the East Coast still face days of delays even as airlines start flying again Monday at major airports that closed for Tropical Storm Irene.

Ground transportation — including New York City’s subways — also started easing back into service Sunday evening, though disruptions remained widespread.

More than 11,000 flights were canceled nationwide over the weekend, and hundreds more will be scrubbed Monday morning, a flight-tracking service said.

There’s no easy way to squeeze all those displaced passengers onto scheduled flights, especially if airports around New York — the nation’s busiest airspace — encounter delays reopening Monday. And ground transport alternatives remain limited, with bus and train service disrupted into Monday as well along the East Coast.

Airlines won’t say how many passengers have been grounded since Irene came ashore in North Carolina on Saturday.

FlightAware, which tracks cancelations, put the total around 650,000, noting that many of the 11,800-plus flights canceled so far were on regional airlines that use small planes. Some travel experts suggested much larger numbers — 1 million or more.

Finding open seats will be especially difficult this week because it’s the last gasp of the summer vacation season.

"We’re coming into the Labor Day holiday weekend, so a lot of those flights are already full," said Todd Lehmacher, a spokesman for US Airways.

Airlines resumed flights Sunday at airports around Washington, Philadelphia and Richmond, Va. But the longer closure of the New York area’s Kennedy, LaGuardia and Newark, N.J., airports means travel delays will continue rippling across the country.

The Port Authority of New York and New Jersey, which operates the three airports, said late Sunday that Kennedy and Newark would open to arriving flights at 6 a.m. Monday, with departures beginning at noon. LaGuardia was to reopen at 7 a.m.

New York City officials said they weren’t sure Sunday evening when mass transit would be fully restored after shutting down Saturday. Some local bus service resumed Sunday, and the subways will resume some service Monday, though officials said commuters should expect long lines and long waits. Downed trees and high water still covered commuter train tracks across the region, according to a temporary transit information website offering periodic updates, www.mta.info.

Airline officials said those disruptions would affect their ability to get back into gear.

"It’s really dependent upon mass transit and the airport being ready to support the start-up," JetBlue CEO Dave Barger told NBC.

Long-distance bus and train companies also saw lingering effects from Irene, which was downgraded early Sunday from a hurricane to a tropical storm as high winds ebbed.

Greyhound scrubbed bus travel between Richmond, Va., and Boston all weekend. A spokeswoman said buses would begin to roll north out of Richmond Monday morning and the company hoped to be running in New York by midday.

Amtrak said trains from New York to Florida will be canceled Monday, as will the car train between Lorton, Va., and Sanford, Fla. Some lines in North Carolina and Florida will be open.

Amtrak said in a statement Sunday evening that many routes south of Philadelphia will resume operation, while it canceled many trains between New York and points north. The railroad said separately that its inspections are revealing problems with wiring and signals, as well as trees blocking the tracks. Passengers with paid tickets on canceled trains can rebook or receive refunds by calling 800-872-7245 or visiting Amtrak.com.

But United, Continental, Delta, American, JetBlue, Southwest, AirTran and US Airways canceled hundreds of Monday-morning flights to the New York and Boston area.

Airlines said passengers should call ahead and make sure they have a confirmed seat before going to the airport, but the 670 flights that FlightAware said airlines had canceled for Monday is a small share of the nation’s daily flights.

Airlines also moved several hundred planes out of the storm’s path to avoid damage, which will further slow the return to normal service.

When blizzards hit the East Coast in December and February, it took some passengers days to get home. That could happen again.

Sara Hesselsweet of Norwalk, Conn., and her family were to fly home Sunday from vacation at Lake Tahoe on the California-Nevada border. After their flight was canceled, American Airlines told her it couldn’t find seats for her, her husband and 2-year-old son until next Saturday.

So the family decided to fly from Reno, Nev., to Dallas and on to Chicago, where they would rent a car to drive back to Connecticut.

"We checked Philadelphia, D.C., Boston, the Carolinas — we couldn’t get a flight anywhere," said Hesselsweet, sitting amid a pile of carry-on bags in the Reno airport.

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AP Airlines Writer Scott Mayerowitz contributed to this report from Reno.